Enable new Systems Manager experience with only one Management Account

Yes, you can use AWS Systems Manager without setting up a new AWS account or registering a delegated administrator account, especially if you're a small operation with only one AWS account. The Systems Manager unified console experience can be set up for a single AWS account and AWS Region without the need for AWS Organizations or a delegated administrator account.

For your scenario, you can use the Systems Manager setup process for a single account, which will create the necessary resources and permissions within your existing management account. This setup process will automatically create and attach the required IAM roles and permissions to your nodes, set up State Manager associations, and create S3 buckets and Lambda functions needed for Systems Manager operations.

The single-account setup includes features like automatic diagnosis and remediation of nodes that aren't reporting as managed by Systems Manager. This can help identify and resolve issues such as network connectivity problems to Systems Manager endpoints.

It's important to note that while using a delegated administrator account is considered a best practice for larger organizations with multiple accounts, it's not strictly necessary for small operations with a single account. You can effectively use Systems Manager's core functionalities within your existing management account without the need for additional accounts or complex organizational structures.

If your needs grow in the future and you decide to expand to a multi-account structure, you can always set up AWS Organizations and designate a delegated administrator account for Systems Manager at that time. For now, you can proceed with enabling and using Systems Manager within your single management account without any additional account setup.
Sources
Setting up Systems Manager unified console for a single account and Region - AWS Systems Manager
Enable Systems Manager for organization-wide use | AWS re:Post
Creating an AWS Organizations delegated administrator for Systems Manager - AWS Systems Manager